1. Field of the Invention
This invention relates to storage tubes, to storage tubes of the variety known as scan converter tubes and, in particular, to a storage target of improved performance characteristics for use in scan converter tubes.
2. Description of the Prior Art
Oscilloscopes using cathode-ray tubes have been used extensively for the observation and analytical study of high-speed transient phenomena, high-speed signals of low repetition frequencies and the like, generally with unsatisfactory results. The advent of storage tubes with higher writing speed has long been awaited in various fields of electronics. In addition to the problem of low writing speed, the conventional storage tubes are disadvantageous in their low resistivity to electron beam bombardment and in their expensiveness.
Storage tubes can be broadly classified into direct-view and non-direct-view types. Included in the latter type is a scan converter tube which finds its principal application in the field of picture processing, being used as a frame memory or the like. In its use as a frame memory, the writing speed of the scan converter tube is required to be only just as high as that required for conversion of an optical image into an electrical signal by a television camera tube. The writing speed of the conventional scan converter tube is insufficient, however, for high-resolution operation wherein the number of scanning lines is doubled. The conventional scan converter tube has the additional problem of so-called "burning" caused by the electron beam and or impairment by soft X-rays generated by the interaction of the electron beam and the field mesh.
The storage target in the scan converter tube comprises, according to a typical prior art example, a silicon substrate, a silicon dioxide storage layer formed on the substrate by thermal oxidation thereof, and a collector electrode of latticed or striped design formed further on the storage layer. The collector electrode functions as such during writing and erasing operations and as a reading electrode during reading operation. The latticed or striped collector electrode is therefore electrically connected to a collector voltage supply circuit and to a reading circuit.
While the storage layer of the prior art storage target is usually formed as aforesaid by thermal oxidation of the silicon substrate, this layer may also be created by other methods such as cathode sputtering or chemical vapor deposition of silicon dioxide or other insulating substance on the silicon substrate. Regardless of the way the storage layer is formed, the crystal lattice constant and thermal expansion coefficient of the silicon substrate differ from those of the storage layer thereon. The storage layer is therefore either noncrystalline or polycrystalline. As a consequence, the thickness of the storage layer has been limited to 2 to 3 microns by reasons of possible cracking and buckling. Because of such small thickness of the storage layer, the capacity of the storage target increases, with the consequent decrease in writing speed.
The non- or polycrystalline storage layer of the prior art storage target also provides a cause for a low secondary emission ratio (number of secondary electrons/number of primary electrons) and, therefore, for low writing speed. The secondary emission ratio is further lowered by the impurity and surface contamination of the storge layer, as such contamination occurs, during the formation of the collector electrode thereon, by surface adsorption of impurities because of its non- or polycrystalline nature. The molecular binding of the storage layer is also low, so that the layer is susceptible to burning upon electron beam bombardment. Furthermore, as a number of levels exist in the energy gap of the storage layer, leakage and impairment by soft X-rays also present problems, setting limits upon zooming operation.
According to another example of storage target heretofore used in a scan converter tube, a collector electrode is formed on a glass substrate. The secondary emission ratio of this second prior art storage target is also very low because its glass substrate is noncrystalline, containing much impurities. As in the preceding example, the secondary emission ratio is further lowered during the formation of the collector electrode on the substrate. The silicon dioxide storage layer is highly susceptible to contamination by alkaline ions, particularly sodium ions, giving rise to problems such as unstable operation, shorter storage time, and variation in the level of scanned areas. It may be stated by way of reference that the secondary emission ratio of the cleavage face of high purity quartz glass in vacuum is approximately 2. Probably, the secondary emission ratio of the above prior art examples is considerably less than 2.
It is possible to lessen the electrostatic capacity of the storage target by increasing the thickness of its glass substrate. As will be later explained in further detail, however, the above statement must be taken in light of the fact that, as the substrate thickness is increased to a certain degree, the electrostatic capacity of the storage target is determined rather by the pitch of the collector stripes or the like and by other factors. Sufficiently high writing speed cannot therefore be attained by this measure. The prior art storage target with the glass substrate, moreover, has the same problem of easy impairment by an electron beam and soft X-rays as that encountered with the first described prior art storage target with the silicon dioxide storage layer.
An equivalent of the storage target in a storage tube of the direct-view type is a storage mesh, which includes a fine metal mesh having formed thereon a storage layer of, typically, magnesium oxide. The magnesium oxide storage layer is formed either by baking magnesium oxide powder or by evaporation of magnesium in an oxidative atmosphere.
Typically, the direct-view storage tube with such a storage mesh has a writing speed of, in terms of frequency, up to about 10 megahertz (100 cm/.mu.s). This limitation is imposed by the low secondary emission ratio of the magnesium oxide storage layer, which is either non- or polycrystalline like the above described prior art storage layers of scan converter tubes.
During writing operation, the cathode of the direct-view storage tube is set at -1800 volts, and the storage mesh at or close to ground potential, so that the acceleration energy of the writing beam is about 1800 electron volts. Generally speaking, the writing beam energy should be so determined as to realize substantially the maximum possible secondary emission ratio.
In the direct-view storage tube of the type under consideration, however, it is necessary to give a certain degree of beam acceleration energy in order to prevent an undesired increase in the spot diameter of the electron beam. The acceleration voltage setting actually employed, therefore, is not the one which will provide the maximum possible secondary emission ratio, so that the writing speed is limited also in this respect. The low writing speed of the direct-view storage tube is further attributable to the poor practical efficiency of the writing beam because of the presence of the storage mesh and a collector mesh in the tube.
Another serious problem that must be taken into consideration in connection with the prior art direct-view storage tube is the thermal impairment of the storage mesh by the electron beam. This is due to the poor binding of the constituent molecules of the non- or polycrystalline storage layer. Moreover, since this storage layer lies on the metal mesh, these components of the storage mesh have a great difference in their thermal expansion coefficient, so that high localized heating takes place by Joule heat upon electron beam bombardment.
A technique known as the "high speed mode" has been employed in connection with the direct-view storage tube of this type, for improving the writing speed through an increase in beam acceleration energy during writing operation. The writing speed can certainly be multiplied by this technique. The problem of burning by the electron beam becomes all the more serious, however, because of the increased beam acceleration energy and increased beam intensity. Furthermore, the acceleration energy is set at a value considerably off the value which will give the maximum secondary emission ratio. The writing speed does not increase so much as the increase in electron beam intensity. The doubling of the beam acceleration energy results in approximately the halving of the deflection zone, to the inconvenience of the viewer. This is due to a decrease in deflection sensitivity caused by the increased beam velocity.
The prior art direct-view storage tube has the additional disadvantage of being expensive, because of difficulties involved in the manufacture of its storage mesh and collector mesh. This has also been an impediment to the widespread use of the storage tube. Thus, the prior art storage tubes of direct- and non-direct-view types alike have the problems of limited writing speed, the burning of the storage media by electron beams, and so forth, all accruing from the noted imperfections of the storage media.